I dont want to argue also,but I would like to point out that I have rang two reputable pond suppliers who stock good quality koi and sell good equiptment and have helped me when setting up and aciieve the results that I want. The statement of facts to them was
Pond sizes 3ft x 8ft x 3ft depth = approx 2000 liters Their response was that IF THE PERSON WHO OWNED THE POND wanted to introduce koi there is no reason not to do so as long as the filteration is to a good standard
I do agree that the pond in question is small and that it would be best put on hold any plans to introduce koi but this is proffesional advice from two sorces.
At the end of the day it the pond owners choice and I did ASK bluechild previosly if it was something he/she intended doing
no hard feelings all information is based on the success achieved with an attempt to pass this on
Bearing in mind that shops are not always the best to give guidance on these matters and that adult Koi are very large fish I would agree with 2010 that the OP's pond is not big enough to house them properly. The size quoted by those shops as being suitable seems well short of what is really needed. 6ft x 6ft x 5ft deep minimum (IMO).